Digital computers which process multi-media signals comprised of moving video images, graphics, etc. require the capability to display such images on 8 bit per pixel (256 color) pseudo color displays.
Currently there are several ways in which this can be accomplished:
(a) The 256 most common colors in the image are chosen, and a color palette is set appropriately. Each pixel is remapped to the closest color in the palette. This has worked quite well except when there have been multiple images on the screen or when animating the image or any time that there is an even color distribution. For animation, the entire procedure must be performed for each frame and requires much preprocessing and run time processing, which results in an unacceptably low frame rate.
(b) All colors are truncated or rounded to a normalized RGB 3:3:2 display. Picture quality using this method has been found to be very poor. Bands occur on color boundaries and subtle features in the image are lost. This method does not work very well on digitized video images.
(c) Resolution is traded off for color accuracy, in a procedure referred to as ordered dithering. The entire image is subdivided into 2.times.2 pixel areas, and the average color is computed for each area. The two colors within the 256 color palette that are closest to the average color are chosen. The closeness of each color to the average color determines the ratio (0%, 25%, 50%, 75% or 100%) of the colors to use. The perceived correctness of the colors is very good, but the resulting image has only 1/4 of the original spatial resolution, and color bands occur on some color boundaries. The picture quality is between the quality achieved in methods (a) and (b) noted above.